There are many situations in which light rays can be used for determining a physical characteristic of a material. For example, it is sometimes desirable to measure the thickness of a layer that is deposited on top of a substrate. That is, when a layer on top of a substrate is being planarized or otherwise partially removed in a polishing process, one may want to determine (directly or indirectly) the remaining thickness so that too much material is not removed. As another example, when a layer is being deposited on a substrate, one may want to determine (directly or indirectly) the deposited thickness so that too much or too little of the layer material is not deposited. It can also be important to determine uniformity of the layer thickness. Thus, the purpose of determining the thickness in some situations may be to determine a desired end point of a manufacturing process or what pressure profile to use across the wafer, and/or a polish time to use for the wafer. In other examples, a physical characteristic such as thickness may be determined for quality control, classification, calibration, compatibility testing, or other purposes.
Chemical mechanical polishing (CMP) is one example of a manufacturing process in which end point determination or real-time thickness monitoring is performed. For example, CMP is sometimes performed on a wafer or other substrate that includes integrated circuits. An integrated circuit is typically formed on a substrate by the sequential deposition of conductive, semiconductive or isolative layers on a silicon wafer. After each layer is deposited, the layer is etched to create circuitry features. As a series of layers are sequentially deposited and etched, the outer or uppermost surface of the substrate, i.e., the exposed surface of the substrate, becomes increasingly non-planar. This non-planar surface presents problems in the photolithographic steps of the integrated circuit fabrication process or the electrical properties of the contact lines or the devices. The deposited layers must be planarized and then polished down to a specified thickness.
CMP is one accepted method of planarization. This planarization method typically requires that the substrate be mounted on a carrier or polishing head. The exposed surface of the substrate is placed against a rotating polishing pad, or a pad that is an axially moving sheet. The polishing pad may be either a “standard” pad or a fixed-abrasive pad. A standard pad has a durable roughened surface, whereas a fixed-abrasive pad has abrasive particles held in a containment media. The carrier head provides a controllable load profile, i.e., pressure, on the substrate to push it against the polishing pad. A polishing slurry, including at least one chemically-reactive agent, and abrasive particles if a standard pad is used, is supplied to the surface of the polishing pad.
The effectiveness of a CMP process may be measured by its polishing rate, and by the resulting finish (absence of small-scale roughness) and flatness (absence of large-scale topography) of the substrate surface. The polishing rate, finish and flatness are determined by many factors, including the pad and slurry combination, the carrier head configuration, the relative speed between the substrate and pad, and the force pressing the substrate against the pad.
In order to determine the effectiveness of different polishing tools and processes, a so-called “blank” wafer, i.e., a wafer with multiple layers but no pattern, may be polished in a tool/process qualification step. After polishing, the remaining layer thickness may be measured at several points on the substrate surface. The variation in layer thickness provides a measure of the wafer surface uniformity, and a measure of the relative polishing rates in different regions of the substrate. One approach to determining the substrate layer thickness and polishing uniformity is to remove the substrate from the polishing apparatus and examine it. For example, the substrate may be transferred to a metrology station where the thickness of the substrate layer is measured, e.g., with an ellipsometer. This process can be time-consuming and thus costly, and the metrology equipment is costly.
One challenge in CMP is determining whether the polishing process is complete or uniform, i.e., whether a substrate layer has been uniformly planarized to a desired flatness or thickness. Many different factors can cause variations in the material removal rate, including variations in the initial thickness of the substrate layer and its properties, the slurry composition, the polishing pad condition, the relative speed between the polishing pad and the substrate, and the load on the substrate. These variations in turn cause variations in the time needed to reach the desired thickness and uniformity. Therefore, these and other properties cannot be determined merely as a function of polishing time.